The invention relates to a method for testing a sample by means of a particle beam microscope, i.e., an ion or electron beam microscope, in which the sample is scanned in a point-wise manner by a focused beam of charged particles for generating imaging signals. The invention further relates to a device for carrying out the method.
Electron microscopy methods of the above-mentioned kind, in which the sample is scanned by a focused electron beam (called REM (Rasterelektronen-mikroskopie) or SEM (scanning electron microscopy) method), have been used for a long time, e.g., in material research. The scanning method is also used in transmission electron microscopy (TEM) as so-called scanning transmission electron microscopy (STEM), in which the scanning electron beam radiates through the sample. STEM tomography, which uses a focused electron beam, allows larger sample thicknesses as compared to the TEM method. The thickness of a carbon-based sample can be up to some micrometers, in the TEM method only some hundred nanometers. As compared to the TEM method, the STEM method, in which an overall dose that is higher by one to two orders is applied to the sample by the point-wise scanning by the electron beam, is disadvantageous in view of a lower dose efficiency. In STEM tomography, a series of data sets is generated in a very time-consuming manner in that the sample is scanned by means of the electron beam at different inclination angles relative to the focused electron beam in a two-dimensional manner by transmission radiation. On the basis of the data sets, which comprise 3D information, images of the interior of the sample can be reconstructed by using tomographic standard methods. It is also time-consuming to obtain three-dimensional imaging data in the so-called serial section method by means of a scanning electron microscope. For this purpose, the surface of a sample is repeatedly scanned by means of a focused electron beam, and after each scanning process the uppermost layer of the sample is removed by means of an ion beam so that a respective new section surface is formed. Thus, different layer images of the sample, which comprise 3D information, are obtained.
In the method described above, the same electron beam dose is applied in each scanning point during scanning by the focused electron beam. As a result, different loads are applied to the volume elements of the sample. In particular volume elements which lie on the axis about which the sample is rotated for adjusting different inclination angles are irradiated several times, which partly leads to very high loads on the sample. A low dose per scanning point would therefore be desirable, also for reasons of a reduced time necessary for generating the data sets. On the other hand, low doses are an obstacle to a high resolution in 3D reconstruction since the signal-to-noise ratio deteriorates.